a) If you are not concerned about a maximally hollow sphere, you could just color the cell as white if the center of the cube is within a sphere of radius (R - 1.5). In other words, 1.5 units less than the target radius. This results, however, in a slightly over-engineered solid, in that there are a few more interior cubes than needed to make sure that each layer is created out of contiguous cubes. The formula for cell E3 becomes:

b) If you want a maximally hollow square, I think that you need to use an iterative formula, filling in a cell if the cell that is 1 further out horizontally and vertically is filled in, but the cell that is 1 further out diagonally is not. Just to simplify this, I would probably put math formulas into just 1/4 of the spreadsheet area, and populate the other 3/4 by setting the formulas to just be mirror images of the math-calculated area.